1. Field of the Invention
The present invention relates to a spatial frequency converting device and an optical system having the same, for example, an optical system having a device for converting spatial frequency characteristics of the optical system, such as a pupil modulation element. More particularly, the present invention relates to fixed-focus optical systems having no focusing mechanism, such as endoscope optical systems, and variable-focus optical systems.
2. Description of Related Art
There has heretofore been known a technique of converting the spatial frequency characteristics of an optical system by using a pupil modulation element to enlarge the depth of field of the optical system, as disclosed, for example, in PCT/US96/01514. In such an optical system, the spatial frequency characteristics of the optical system are made constant over a wider range than the normal depth of field by the pupil modulation element. However, the use of a pupil modulation element gives rise to problems. That is, the spatial frequency response of the optical system lowers in the frequency range of from a high-frequency region to an intermediate-frequency region, and a phase shift is produced at each frequency.
In PCT/US96/01514, therefore, signal processing is executed to increase the spatial frequency response of the optical system in the frequency range of from a high-frequency region to an intermediate-frequency region and to correct the phase shift at each frequency.
Thus, because the spatial frequency characteristics of the optical system are constant over a wide depth of field, an image of high quality can be obtained over an enlarged depth of field by fixed signal processing irrespective of the distance to the subject.
FIG. 16 is a sectional view of an optical system according to the prior art. Lens data is shown in Table 1 (shown later). In FIG. 16, a pupil modulation element 1 as disclosed in PCT/US96/01514 is applied to a fixed-focus optical system.
The focal length of the optical system is 1.603 millimeters, and the F-number thereof is 4.4265.
The pupil modulation element 1 has a free-form surface on one side thereof (surface No. 6). The free-form surface has a configuration given by z=-0.08(x.sup.3 +y.sup.3). It should be noted that the optical axis is defined as a z-axis, and coordinate axes perpendicularly intersecting the z-axis are defined as x- and y-axes. The unit is millimeter. The aperture stop 2 (surface No. 7) is a square aperture, each side of which is 0.9 millimeters long. The directions of the sides of the aperture stop 2 are coincident with the x- and y-axis directions of the pupil modulation element 1.
FIGS. 18 to 20 show the results of calculation of the spatial frequency characteristics of the optical system shown in FIG. 16 on the optical axis [(x,y)=(0,0)] and at an image height of 1.5 millimeters [(x,y)=(0,1.5)] performed with simulation software Code-V (trade name). In FIGS. 18 to 20, curves A and B represent the spatial frequency characteristics in the y- and x-axis directions, respectively, on the optical axis, and curves C and D represent the spatial frequency characteristics in the y- and x-axis directions, respectively, at an image height of 1.5 millimeters. The x- and y-axis directions are set coincident with the coordinates of the pupil modulation element 1.
FIG. 18 shows the spatial frequency response of the optical system shown in FIG. 16 in a case where the subject distance is 7 millimeters. Phase components in this case are shown in Table 2 (shown later).
FIG. 19 shows the spatial frequency response of the optical system shown in FIG. 16 in a case where the subject distance is 13.5 millimeters. Phase components in this case are shown in Table 3 (shown later).
FIG. 20 shows the spatial frequency response of the optical system shown in FIG. 16 in a case where the subject distance is 25 millimeters. Phase components in this case are shown in Table 4 (shown later).
For comparison, an optical system that does not use a pupil modulation element is shown in the sectional view of FIG. 17. Lens data is shown in Table 1. It should be noted that the optical system shown in FIG. 17 uses a plane surface as the surface No. 6 of the optical system shown in FIG. 16.
FIG. 21 shows the spatial frequency response of the optical system shown in FIG. 17 in a case where the subject distance is 7 millimeters. Phase components in this case are shown in Table 5 (shown later).
FIG. 22 shows the spatial frequency response of the optical system shown in FIG. 17 in a case where the subject distance is 13.5 millimeters. Phase components in this case are shown in Table 6 (shown later).
FIG. 23 shows the spatial frequency response of the optical system shown in FIG. 17 in a case where the subject distance is 25 millimeters. Phase components in this case are shown in Table 7 (shown later).
It will be understood by comparing FIGS. 18 to 20 and Tables 2 to 4 that the spatial frequency characteristics of the optical system are approximately constant independently of the distance to the subject. In addition, the spatial frequency response does not reach zero.
Comparison of the FIGS. 21 to 23 and Tables 5 to 7 reveals that the spatial frequency characteristics change, depending on the distance to the subject. In FIGS. 21 and 23, the spatial frequency response reaches zero. Accordingly, in the ordinary optical system shown in FIG. 17, the depth of field is smaller than the region of from 7 millimeters to 25 millimeters.
Thus, the pupil modulation element 1 allows the spatial frequency characteristics to be constant over a wider range than the depth of field of the ordinary optical system.
The pupil modulation element according to the prior art is always in a state of performing spatial frequency characteristic conversion. Therefore, the depth of field is constant at all times and cannot be changed by the user. In addition, it is assumed that the pupil modulation element is used in a fixed-focus optical system. Therefore, in a variable-focus optical system, the amount of conversion of spatial frequency characteristics changes with changes in the focal length of the optical system. Accordingly, the depth of field cannot be enlarged with a fixed spatial frequency characteristic restoring device (signal processing).